1. Field of the Invention
This invention relates generally to a dc-to-dc converter and more specifically to such a converter which is provided with an improved power supply arrangement by which undesired heat generation in the converter can effectively be reduced.
2. Description of the Prior Art
In order to obtain a stable dc power output without an undesirable increase in the number of electronic components, it is a known practice to implement dc-to-dc conversion of a rectified but unregulated ac line voltage through the use of a dc-to-dc converter. This kind of power supply circuitry is currently employed in a variety of electronic equipments.
Before describing in detail the present invention a known dc-to-dc converter will be discussed with reference to FIG. 1.
Input terminals 11 and 12 of a dc-to-dc converter 10 is supplied with a full-wave rectified ac line voltage (100 V by way of example). A capacitor 14, coupled across the input terminals 11, 12, operates as a capacitor filter. As shown, a transformer 16 is provided with three primary windings L10, L11, L12, and a secondary winding L20. The winding L10 and a switching transistor 18 are coupled in series across the input terminals 11 and 12. The transistor 18 is selectively rendered conductive (viz switched on and off) by a train of pulses applied thereto from a transistor switching controller 20. An ac voltage developed across the secondary winding L20 is rectified at a rectifier 22 whose dc outputs are applied to the transistor switching controller 20 and also applied to output terminals 24, 26 of the dc-to-dc converter 10. The transistor switching controller 20 changes the pulse duration of the output pulse thereof in accordance with the dc output applied to two control inputs thereof from the rectifier 22, thereby controlling the dc output derived at the output terminals 24, 26. The transistor switching controller 20 is known in the art and is prepared by, for example, combining two monolithic ICs such as .mu.PC1093J and .mu.PC1094C manufactured by the NEC Corporation. A diode 70, coupled in series with the primary winding L12 between the input terminals 11 and 12, is provided for releasing electromagnetic energy induced when the transistor 18 opens.
An arrangement 50, enclosed by a dashed line, supplies the transistor switching controller 20 through two dc-power supply inputs with dc power. The power supply arrangement 50 comprises a three-terminal voltage regulator 52, two resistors 54 and 56, a Zener diode 58, a diode 60 and the primary winding L11. More specifically, the resistor 54 and the Zener diode 58 are connected in series between the input terminals 11, 12. On the other hand, a series circuit, consisting of the resistor 56, the diode 60 and the winding L11, is connected across the Zener diode 58. The three-terminal voltage regulator 52 has its input terminal "IN" coupled to a junction 55 between the components 54, 56 and 58, and its output terminal "OUT" coupled to the "+" input of the controller 20. The ground terminal "GND" of the voltage regulator is coupled to the "-" input terminal of the dc-to-dc converter 10. Merely by way of example, the three-terminal voltage regulator 52 may comprises a .mu.PC7805H IC chip which is manufactured by the NEC Corporation.
When the dc-to-dc converter 10 is first put into operation, the pulse duration of each pulse outputted from the controller 20 is very narrow and increases to the desired width over a finite period of time. Therefore, during the initial period of operation of the dc-to-dc converter 10, dc power required by the controller 20 is relatively small and can be readily supplied to the controller 20 through the resistor 54 and the three-terminal voltage regulator 52. However, as the pulse duration of each of the pulses from the controller 20 becomes longer and the dc output of the converter 10 increases, the dc power supply through the resistor 54 encounters a limit. In such a case, additional dc power is supplied to the three-terminal voltage regulator 52 through the winding L11, the diode 60 and the resistor 56. The resistor 56 and the Zener diode 58 prevent an excessive voltage from being applied to the three-terminal voltage regulator 52.
As mentioned previously, in order to obtain a stable dc output power without substantially increasing the number of electronic components, a rectified ac line voltage (100 V for example) is applied to such a dc-to-dc converter as shown in FIG. 1. For the convenience of description it is assumed that (a) the input voltage of the transistor switching controller 20 should be maintained 15 V and (b) the rated input voltage of the three-terminal regulator 52 is 35 V and (c) the output voltage of the regulator 52 should be maintained at 15 V. It is further assumed that the maximum allowable input voltage to the input terminals 11 and 12 is the instantaneous value (about 140 V) of the ac line voltage plus several tens of volts. Therefore, in order that the three-terminal voltage regulator 52 is not rendered inoperative due to the input of a voltage exceeding the maximum allowable value, the breakdown voltage of the Zener diode 58 is set to about 30 V. The parameters of the resistors 54, 56 and the ac voltage across the winding L11, should carefully be determined to minimize power consumption at the resistors 54, 56 and at the Zener diode 58 while retaining the power supply of 15 V to the input of the controller 20. However, this prior art has encountered the problem that even the careful adjustment of the parameters of the components are unable to eliminate an undesired amount of power consumption and generation of heat at the resistor 56 and the Zener diode 58.